1. Field of the Invention
This invention relates to the fabrication of integrated circuit devices and more particularly to a method of local oxidation using a multiple LOCOS process in the fabrication of integrated circuits.
2. Description of the Prior Art
Local oxidation of silicon (LOCOS) is the conventional lateral isolation scheme. The conventional local oxidation process is described in VLSI Technology, International Edition, by S. M. Sze, McGraw-Hill Book Company, NY, N.Y., c. 1988 by McGraw-Hill Book Co., pp. 473-474. Referring to FIG. 1, a layer of silicon nitride 14 is deposited over a pad oxide 12 overlying a silicon substrate 10. The pad oxide is a thin thermal oxide which allows better adhesion between the nitride and silicon and which acts as a stress relaxation layer during field oxide formation. The nitride and oxide layers are etched to leave openings exposing portions of the silicon substrate where the local oxidation will take place. A boron channel-stop layer is ion implanted into the isolation regions. The field oxide is grown within the openings and the nitride and pad oxide layers are removed. This completes the local oxidation.
However, as the silicon nitride opening between active device regions is reduced to the submicrometer regime, a reduction in field oxide thickness within the opening has been observed. Referring to FIG. 2, field oxide regions 16, 18, and 20 are not of the same thickness. The thickness T of the field oxide regions is largest for the wider opening and smallest for the narrower opening. That is, T.sub.16 &lt;T.sub.18 &lt;T.sub.20. This thinning of the field oxide region will lead to substantially reduced field threshold voltage and additional field edge leakage current. Thus, the field oxide thinning effect will be the barrier to scaling down isolation width in ultra large scale integrated circuits (ULSI).
U.S. Pat. Nos. 5,061,654 to Shimizu et al and 5,128,274 to Yabu et al describe methods for purposefully making field oxide regions to be of different thicknesses. U.S. Pat. No. 5,173,438 to Sandhu shows a method for adjusting ion implantation to achieve different field oxide thicknesses. The purpose of the present invention is to achieve the same field oxide thicknesses for different sized openings.